The naturally occurring C-nucleoside formycin has demonstrated a broad range of biological properties (e.g., antitumor) which have emanated from its isomeric relationship with adenosine. To date, the complete chemotherapeutic potential of formycin has been limited by its rapid deactivation in vivo by adenosine deaminase. A number of studies have arisen to account for this propensity for adenosine deaminase. This led to a recent hypothesis which suggested that the confirmational preference (syn or anti) of the ribofuranose moiety in formycin was the responsible feature. However, this assumption has been discarded and replaced by one which says that the pi-bonding arrangement in and stereochemical environment around the heterocyclic base unit of formycin are the controlling factors in the adenosine deaminase affinity for this nucleoside. Therefore, in order to verify this conclusion, several analogs of formycin (namely, 7-amino-3-(Beta-D-ribofuranosyl)isoxazolo(4,5-d)pyrimidine (2) and 7-amino-3-(Beta-D-ribofuranosyl)isoxazolo(4,3-d)pyrimidine (3), 8-amino-1-methyl-3-(Beta-D-ribofuranosyl)-1H-pyrazolo(3,4-g)quinazoline (4), 8-amino-3-(Beta-D-ribofuranosyl)-1H-pyrazolo(3,4-g)quinazoline (5), and 4-amino-7-(Beta-D-ribofuranosyl)pyrazolo(3,4-e)(1,3)oxazine (6), whose heterocyclic bases possess specific pi-bonding patterns with and without attendant steric probes, will be prepared and biologically evaluated against adenosine deaminase. These compounds will also be assayed as substrates for adenosine kinase as an indicator by their ability to be converted into their corresponding nucleotides, a prerequisite for formycin biological involvement. In this latter regard, 2-6 will be tested as antitumor agents, the results of which will be correlated with the adenosine deaminase and adenosine kinase data for 2-6. Finally, it is anticipated that this study will guide the future design of chemotherapeutic agents derived from formycin and will also enlighten the structural demands adenosine deaminase and adenosine kinase require of a molecule in order for it to be a substrate.